Generally, lithium ion secondary batteries are widely used as a power source for small-sized electronic devices such as notebook computers and cellular phones. Furthermore, since the lithium ion secondary batteries have high power, high capacity, and light weight when compared to other secondary batteries, they are also being used in hybrid and electric automobiles.
Lithium ion secondary batteries used as a power source of automobiles should satisfy safety and reliability requirements under the severest conditions. There are a plurality of test categories for safety, among which the three most extreme test categories are for puncture, collapse, and overcharge.
The puncture and collapse tests are performed to estimate secondary battery-related damage in the event of a car accident, and are thus very important test categories for safety. Specifically, in tests performed under extreme conditions such as a nail penetration test and a collapse test, the battery should not excessively increase in temperature.